KVM: nVMX: Move control field setup to functions

Move some of the control field setup to common functions. These functions will
also be needed for running L2 guests - L0's desires (expressed in these
functions) will be appropriately merged with L1's desires.

Signed-off-by: Nadav Har'El <nyh@il.ibm.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>